How can I make an object invisible when I render in Cycles?
These are some of the options to make an object not render in Cycles.

To make the object invisible/unrenderable


Disable the object for rendering (turn off the camera icon)


Move the object you don't want to a layer that is not renderable


Give it a transparent shader:




To make the object invisible to camera but keep the shadows and interaction with other objects (reflections, etc)


Restrict the visibility for camera:


Using Nodes: use a light path so the object is invisible to the camera only:




To make the object transparent as alpha channel:


use a holdout shader so it renders as transparent


Place the object on a different layer and use that layer as mask:


Use a Pass index for the object and use that on the compositor to subtract the object from the final render:


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Similarly, you can assign a pass index to the material and use that as alpha on the compositor:


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For the last three options the Transparent Background option has to be enabled: